linux-kernel - Re: [PATCH 1/2] kmemcheck v3

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Message-ID: <20080208132032.GA6115@one.firstfloor.org>
Date:	Fri, 8 Feb 2008 14:20:32 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Vegard Nossum <vegard.nossum@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Richard Knutsson <ricknu-0@...dent.ltu.se>,
	Christoph Lameter <clameter@....com>
Subject: Re: [PATCH 1/2] kmemcheck v3

On Fri, Feb 08, 2008 at 01:18:37PM +0100, Vegard Nossum wrote:
> On 2/8/08, Andi Kleen <andi@...stfloor.org> wrote:
> > Your assumption that only the string instructions can take
> > multiple page faults seems a little dangerous too.
> 
> Yes, this is true. I cannot guarantee that there are no other
> instructions that could access more than one memory location but only
> take one page fault. However, since the kernel does boot, we at least
> know that these instructions are not very frequently used. (If you
> know of any other instructions we might be missing, I'll be happy to
> know about it!)

Pretty much all in the right circumstances.

e.g. consider a segment reload in tracked memory.

Also there are various instructions which do all kinds of complicated
things internally; like IRET or INT: often with many memory accesses.
Just page through a instruction manual and look at the pseudo code
describing what the various instructions do.

> 
> There is also the point that if kmemcheck doesn't handle all the
> faulting addresses, it will simply fault again and again, without
> making any progress. I mean, it won't go unnoticed for very long :-)
> 
> This is also why we depend on M386 and !X86_GENERIC, to avoid those
> MMX, etc. instructions, as we have no support for those currently

I would not expect problems from MMX/SSE here (except for the generic
ones all instructions have) 

-Andi

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